The spine is formed of a column of vertebra that extends between the cranium and pelvis. The three major sections of the spine are known as the cervical, thoracic and lumbar regions. There are 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae, with each of the 24 vertebrae being separated from each other by an intervertebral disc. A series of about 9 fused vertebrae extend from the lumbar region of the spine and make up the sacral and coccygeal regions of the vertebral column.
The main functions of the spine are to provide skeletal support and protect the spinal cord. Even slight disruptions to either the intervertebral discs or vertebrae can result in serious discomfort due to compression of nerve fibers either within the spinal cord or extending from the spinal cord. Disruptions can be caused by any number factors including normal degeneration that comes with age, trauma, or various medical conditions. If a disruption to the spine becomes severe enough, damage to a nerve or part of the spinal cord may occur and can result in partial to total loss of bodily functions (e.g., walking, talking, breathing, etc.). Therefore, it is of great interest and concern to be able to treat and correct ailments of the spine.
When conservative efforts fail, treating spinal ailments very often includes one of or a combination of spinal fusion and fixation. Generally, spinal fusion procedures involve removing some or all of an intervertebral disc, and inserting one or more intervertebral implants into the resulting disc space. Introducing the intervertebral implant serves to restore the height between adjacent vertebrae (“disc height”) and maintain the height, and/or correct vertebral alignment issues, until bone growth across the disc space connects the adjacent vertebral bodies. Resection of ligaments and/or boney elements from the affected spinal area is also common in order to access the disc space and/or decompress impinged nerve or spinal cord tissue.
Fixation systems are often surgically implanted during a fusion procedure to help stabilize the vertebrae to be fused until the fusion is complete or to address instabilities (either preexisting or created by the fusion or decompression procedure itself). Fixation constructs of various forms are well known in the art. Most commonly, the fixation construct is a plate anchored to the anterior column with multiple bone anchors or a posterior fixation construct including multiple anchors and a connecting rod anchored to the posterior elements of the spine. For a posterior fixation construct the anchors (typically pedicle screws) are anchored into the pedicles of each vertebra of the target motion segment. The pedicle is a dense, strong, stem-like structure that projects from the posterior side of the vertebral body. The anchors are then connected by a fixation rod that is locked to each anchor, thus eliminating motion between the adjacent vertebrae of the motion segment. The fixation anchors utilized in posterior fixation constructs generally include an anchor shank and a rod housing. The rod housing includes a pair of upstanding arms separated by a rod channel in which the fixation rod is captured and locked. When constructing the posterior fixation construct the surgeon must align and seat the rod in the rod channel. This can be a challenge as it requires the rod channels of adjacent rod housings to be generally aligned, or alternatively, the rod must be bent to fit.
In addition to simply stabilizing the spine, components of the fixation system can also be utilized to manipulate the positioning of the vertebrae to achieve the desired alignment before movement is arrested. That is, applying a directional force to the anchor shank through the attached housing, for example, via minimally invasive guides, reduction tools, or other instruments that are commonly engaged to the housing and extend out of the patient, causes the associated vertebra to translate or rotate in the direction of the force, thus allowing the surgeon good control to manipulate the relevant vertebrae into a desired position. However, doing so typically causes the rod housings to move relative to each other. Thus, achieving the desired correction (realignment) of the vertebrae while also aligning the rod channels of the housings to effectively seat a rod is a significant challenge and can create difficulties and delays during the surgery.